Conventionally, inorganic electroluminescent devices have been in use as a planar light source. Because an alternating high voltage is needed for driving the device and blue light emission is difficult, a difficulty has been involved in full colorization based on the three prime colors of RGB.
On the other hand, electroluminescent devices using organic materials have been extensively investigated up to now. For instance, reports have been made on the use of single crystal anthracene or the like as a fluorescent organic compound (Patent Document 1: U.S. Pat. No. 3,530,325 specification), a combination of a hole transport layer and an emission layer (Patent Document 2: JP-A 59-194393), a combination of a hole transport layer, an emission layer and an electron transport layer (Non-Patent Document 1: Japanese Journal of Applied Physics), and the like.
Organic electroluminescent devices should meet requirements for energy conversion efficiency, luminous efficiency and stability of light-emitting materials. The above-mentioned organic electroluminescent devices are not satisfactory with respect to these characteristic properties and thus, further improvements are demanded.
With a full color display, light-emitting materials for red, green and blue constituting the three prime colors are necessary, with the attendant problem on color purities thereof. It is known that currently known organic electroluminescent devices are not satisfactory with respect to the red light emission efficiency.
To cope with this deficiency, there has been developed an organic electroluminescent device wherein blue or bluish green light emission from an organic light-emitting material is subjected to color conversion with a fluorescent dye to emit red light (Patent Document 4: JP-A 3-152897).
In the technique of this Patent Document 4, a difficulty is involved in that because blue or bluish green light generated from an organic light-emitting material is absorbed and only a little number of dyes having a red fluorescence is known, blue to red color conversion is carried out by one step. In the technique of the Patent Document 4, although a stepwise technique using a plurality of dyes is adopted, this technique has a problem in that the emission efficiency of device lowers.
Further, taking these techniques into account, there has been developed an organic electroluminescent device wherein an organic light-emitting material capable of generating light in a bluish violet region and a fluorescent dye absorbing light in the bluish violet region (Patent Document 5: EP-A 1067165).
In the technique of this patent document 5, because a dye having absorption of light in a bluish violet region and also having a red fluorescence is used, conversion to red is possible by one step and its efficiency can be improved over the case using the two-step conversion. However, this method makes use of conversion of bluish violet into red and thus, a lowering of emission efficiency is not avoidable. In addition, when using conventional organic light-emitting materials, an emission intensity after conversion into red is not always satisfactory.
For organic light-emitting materials of organic electroluminescent devices, stable light-emitting materials having excellent charge transportability have been demanded.                [Patent Document 1]            U.S. Pat. No. 3,530,325 specification            [Patent Document 2]            JP-A 59-194393            [Patent Document 3]            JP-A 63-295695            [Patent Document 4]            JP-A 3-152897            [Patent Document 5]            EP-A 1067165            [Non-Patent Document 1]            “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)”, Japan Society of Applied Physics, Corporation Aggregate of Japan, 1988 Vol. 27, pp. L269-L271